text:
As a contractor for an FBO, is it okay for us to transmit Internet signals right above the tarmac?
I have been looking for the FAA Regulations on this with no luck, we are contracted IT with a local FBO and they want to provide internet to all their hangers. The easiest way would be to put a line of sight connector for the network so we can pass the internet over it, HOWEVER it will shoot over the TARMAC and I am not sure if FAA prohibits that. It is only for ground operations the Airplanes won't be using it but I just wanted to cover myself before I did it.

Won't the internet service be interrupted every time a plane lands?  What if (Murphy's law) a plane stops at Just The Right Spot?

Well it would be shooting about 15 ft above the largest planes that we can take at the airport

I think this would fall under FCC regulations rather than FAA regulations. I don't think the FAA even regulates aircraft-to-aircraft transmissions; that's the FCC's job. By the way, keep in mind that airplanes will occasionally be flying 15 feet above the ground, too; any space above a runway is space where aircraft occasionally go.

15 feet above sounds good; momentary interruptions shouldn't be a problem.  Now about helicopters... :-)

While not an authoritative answer, I know many FBOs have been installing and expanding their WiFi coverage, (many pilots like to be able to file and brief from their iPads in the cockpit).  I've never heard of any concerns or limitations.  Also, many smaller airports are near neighborhoods that are absolutely saturated with multiple WiFi sources in every single household.

I didn't think about it falling under FCC, and the specific spot that we will be crossing is only where planes/helicopters land to load passengers so I am not worried about the planes being off the ground at that spot, although I did forget about Helicopters

There are no FAA restrictions on setting up WIFI transmitters and receivers at an airport in the United States.
WIFI transmissions do not interfere with the frequencies used by airplanes for operational purposes.


text:
Can the back tail of an airplane move the neutral point backwards?
I'm currently in the process of designing a prototype aircraft and since I don't have very much experience with this I have the following question: I know that the center of gravity of an airplane should be located in front of the neutral point (aerodynamic center) so it is self stabilizing. The problem with my design is that the main wings have to be in front of the center of gravity. To counteract that, can I design the back wings in a way so that the neutral point is moved behind the CoG?

Yes you can, the tail volume needs to be such that the neutral point shifts aft of the CoG. This answer has some details - positive lift from the tailplane, but it must never stall!
More details about tailplane design in the answers to this question. The absolute safest config at a wide range of CoG is the CoG in front of the Centre of Pressure of the main wing, and a downforce from the horizontal tail.

There are three approaches to this need.  First, you could build what amounts to a tandem wing.  If you check the CoG location for a Rutan Quickie or its many derivatives, I'm pretty sure you'll find it's between the front (lower) and rear (upper) wings.  Likewise, a Flying Flea (aka Pou de Ciel) has the CoG much further aft, relative to the forward/upper wing, than could be the case with a conventional layout.
The other is to build a "lifting tail" proportioned like some of the contest models of the 1940s to 1960s (and a few, like hand-launch gliders, even to the present day).  These have very little decalage, often zero, sometimes even negative -- but by airfoil choice, the designer ensured that the forward surface always flew at a higher coefficient of lift than the rear, so that any increase in speed resulted in a nose-up moment.  These designs are characterized by CoG locations near or even slightly behind the trailing edge of the wing.
The final, and most modern approach, is active control.  A number of modern high performance aircraft (warplanes, generally) are aerodynamically unstable, to the point where a human pilot cannot maintain control if the computer management fails.  They depend on a computer stability system (generally combined with fly-by-wire) to manage the surfaces to simulate static stability, while keeping the ability to respond to control inputs at rates that would be impossible with a conventionally stable aircraft.

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text:
Has there been any research on plasma-producing planes?
Many years ago I read in an expository scientific journal[1] an article about what the future of high speed aviation could be. It said that there were studies on airplanes capable of "plasma-izing" the air in front of them to get better lift and better aerodynamics.
Did they dream this up? Was there real research and, if so, did anything come out of it?
I've tried searching online, but the only things I've found are a bit too technical for me. From what I've been able to understand they are about using plasma generation in place of the control surfaces thus reducing vortices on wings.
[1] (one of those publications aimed at the divulgation of science to everyday people, not just for scientists)

I rejected the edit to the title because the suggested title was about planes replacing their control surfaces with plasma, that is exactly what this question is not about, as clearly explained in the question itself.

+1 For "divulgative." Didn't know that was even a word. Was going to change it to the more common "expository," but I liked it, so I left it :)

Would you possibly be able to find the article? You may be talking about Electrohydrodynamics in relation to Ionocraft which is a broad topic. A lot of aviation research was done in the 60's and 70's and there were many dead end research projects that never actually worked but info was published about their findings none the less.

Or are you talking about this?

@Dave that's exactly one of the articles I found myself and that looks fully focused on control surfaces, that as I wrote in the question has nothing to do with what I'm searching for. I can't find the original article I because it was ages ago and that journal have been long lost

@motoDrizzt ok, you may want to try and find something at least along the lines of what you are looking for which will greatly help those on this site to answer your question. From an aviation standpoint this question is very broad with out a reference. Post what you have found that is too technical and we may be able to help make sense of it.

I went ahead and submitted an edit changing "divulgative" to "expository". I haven't found it in any dictionaries, which means that regardless of how much of a "valid" word it is, people are going to have trouble figuring out what it means.

@TannerSwett Party pooper. I was itching to use it on Scrabble! :)

I have been long wondering this exact same thing. I saw some research that claimed electrifying/plasmifying the air on the leading edge of a wing resulted in insane increase in lift and reduction of drag. But that was years ago and I haven't heard anything since.

Here is a demo showing how a plasma actuator changes the airflow on a wind turbine. https://www.youtube.com/watch?v=mVJjn1pt08g

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text:
How do pattern and overflight altitudes work for airports underneath Class C airspace?
Lowell City Airport (24C) has an elevation of 700 feet MSL. So normally, the pattern altitude is going to be 1,700 feet, and the altitude for overflying the pattern is going to be 2,200 feet.
However, 24C Lowell is located underneath Class C airspace which starts at 2,000 MSL, so you can't fly at 2,200 without talking to Grand Rapids.
What should I do if I'd like to fly over the field? I can think of a few options:

Maybe 24C Lowell has a non-standard pattern altitude, like 1,400, which would allow me to overfly at 1,900. (How could I find this out using official sources?)
Split the difference between pattern altitude and the Class C. If the pattern altitude is 1,700 and the Class C starts at 2,000, maybe I can overfly at 1,900.
Establish communications with Grand Rapids Approach, then go ahead and overfly at 2,200 while monitoring both Approach and CTAF at the same time.
Just avoid overflying the pattern, and fly around it instead.

When in doubt, I'm going with option 4. But what should I know about the other options?
I do have some particular questions about option 3. If I do choose to contact Grand Rapids Approach and climb to 2,200 to overfly, then I'll have to monitor both Approach and CTAF at the same time, right? Can most small airplanes even do that?

I may be wrong, but you seem to be asking how to overfly the field without entering class C and/or talking to ATC. If that's correct, you might like to say explicitly what limitations you have. Otherwise you're likely to get a bunch of answers saying "just talk to ATC".

Why do you want to overfly the field in the first place? Determining the winds/pattern? Tourism? Just passing through?

@Pondlife If there aren't any downsides to talking to ATC, I'll definitely do that, but I'm not quite sure how that would work; I edited my question to ask about that. Plus, if Lowell did have a non-standard pattern altitude which I could overfly at 1,900, that would mean that talking to ATC is totally unnecessary (...or would it?)

@Stephen Probably either to look at the wind sock, or to enter the pattern from the wrong side.

A non-standard pattern altitude would be in the Chart Supplement (formerly A/FD). Nothing is listed, so it's the expected 1700ft.
Overflight is 500ft above pattern altitude to maintain proper separation. Any closer than this (aside from formation flight) is unsafe and could get you violated. Don't do that.
There's no need to monitor CTAF while you're talking to ATC. They will tell you when to switch over. Have that freq preset on your other radio, though, since you'll probably be a lot closer than you're used to for making your first call and won't want to waste time fumbling with charts and knobs.
Going around doesn't seem to apply if your intent is to land there.

Overall, if you're operating in an area with Approach Control, use it! That isn't limited to the primary airport or even within class B/C airspace; they handle everything within a certain distance (at least 20nm in this case) from the primary, and they'd generally rather know what all those blips on their radar are going to do next than be surprised by sudden conflicts.
Rather than monitoring CTAF, I would monitor GRR's ASOS; it's only 9nm away, which should be close enough to decide Rwy 12 vs 30 at 24C. Once ATC does switch you to CTAF, just make your first call and if folks are using the other, they should pipe up quickly--and loudly!
If you're coming from the northeast and landing 30, I'd forget the pattern, turn south early and then do a long straght-in approach. If coming from the southwest and landing 12, turn north early for the same reason. If you really want to see the windsock or field condition, or you just don't like straight-in approaches, or there's traffic in the pattern, you can enter on upwind instead. Note that upwind is on the opposite side from downwind, not overflying the runway as some books incorrectly show it.
Many airports under shelves like this will have a right pattern on one runway specifically to allow entering 45 to downwind from the "outside" both ways. Unfortunately, that's not the case at 24C.

Just talk to the controllers. We live under a Class B with several underlying Class C or D fields. They'll talk to you. They'd rather know what you're planning to do than not, keeps everyone safer.

From the standpoint of the GR Class C airspace, your only choices are to go around Lowell or call GR Terminal for clearance into Class C to go over it, if you want to be legal overflight-wise.
Me, I generally avoid terminal airspace when at all possible when piddling around VFR, so it's a no brainer to just deke around it.

"I think you're talking about Lowell aren't you?" Yep, my mistake. Thanks for catching that! I've edited the question.

Got your ADS-B Out installed yet? Make it easier for all those folks you're not talking to "see" you.

I'm in Canada.  Not being mandated here.

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text:
What are the consequences of an out of balance helicopter?
(Aside from crashing, injury, and possibly death.)
Airplanes which are out of balance will have either poor stall recovery or noses which are too heavy to lift.
What are the consequences of a helicopter which where the CG is outside the envelope?

Is it simply that the rotor will reach its stops and thus there is insufficient command range in all directions?
Or are there subtle effects such as when an aft-CG in an airplane prevents recovery from a stall?

When you say “out of balance” you just mean a CG which is outside the envelope?  On helicopters rotors can also be out of balance. Also note that being prevented from receiving from a stall IS a problem of insufficient control authority.

Yes, the helicopter and not the blades. I'll update the question. I'll disagree that the aft-CG is insufficient control authority in the sense that the question means, although you are, of course, technically correct.

A C of G out of range on a helicopter will cause it to hover with an appropriate slant, whatever it takes to get the C of G under the rotor head, and if the C of G is displaced far enough, yes you would hit the blade flapping limit stops as you have full cyclic input to try to get the thing to hover in one place, and from there it would just keep drifting sideways.
If you have an articulated rotor that can tolerate blade flapping to the physical limits that is.
If it's a teetering rotor system like on a Bell or Robinson product, the same effect will occur, but the stops that are contacted are the teetering stops inside the rotor head that rest against the mast, and the result is "mast bumping", and this will cause the mast to snap off and the machine becomes a falling object. Hopefully while you're still just a couple feet in the air.
So, say you had a huge lead weight on the tail of the machine and you tried to lift off into a hover.
With an articulated rotor, say a Hughes 269/300, the thing would start to lift into the hover with the nose pointing up, and you would eventually have full forward cyclic in, and that wouldn't be enough and the machine would immediately start to fly backwards because even with the rotor tilted as far forward as it will go with the blades running at the swashplate flapping limit and possibly hitting the flapping stops, the Cof G is still aft of the rotor axis so the rotor disc can't get level, so back you go and at some point you probably crash.
With a teetering rotor, say a Bell 47, the thing would start to lift off into the hover with the nose pointing up, and the moment you applied enough forward cyclic, in trying to stop any backward drift, to make the bump stops in the rotor head contact the mast, there'd be a vibration, a big bang, the machine would drop to the ground, and the entire rotor assembly would fly off for a second or two on its own flight powered by its rotating inertia, like a big boomerang that hopefully doesn't come back, and land nearby.

"like a big boomerang that hopefully doesn't come back" <-- that was good for a sensible chuckle!


text:
Is landing gear-up against the regs?
Typically, landing a plane gear-up results in costly damage, which easily triggers the NTSB reporting requirement. However, as a glider pilot I can easily see how landing a glider gear-up would cause no more damage to the plane than a grass stain.
And yet it seems to me that landing gear-up is a somewhat egregious pilot error,  about which the FAA and my insurance company might eventually like to know. If for no other reason, the FAA/NTSB probably have a vested interest in tracking if gear-up prevention techniques are working.

Or maybe not. If I draw an analogy to cars, if I have a million little fender-benders getting my car in and out of the garage, that's surely no one's business but my own.

Landing gear up unintentionally is what’s called a “self punishing act”.  Illegal or not, why add insult to injury?

@KennSebesta: Two words - "metal fatigue".

Has this happened unintentionally on a commercial airliner? Are they supposed to be down when you ditch or land in a corn field?

Half asleep, for a few seconds I understood the title as landing the aircraft with the gear pointing upwards to the sky.

@Mazura Usually they should be up if you're ditching or landing in a corn field. In any case, you should be covered by 14 CFR 91.3 (b). Unless you're ditching or landing in a corn field for fun, in which case you probably have bigger problems.

Please clarify - is the gear-up landing by mistake, presumably an oversight,  and not an intentional choice as a result of equipment failure or other emergency?

@Criggie I'm asking about mistakes on public runways.

Note that you can reverse downvotes within 5 minutes, or if the answer has been edited since you voted.  For most viewers the chosen answer will be at the top.

Why would landing an aircraft be difference from parking a car, except in degree?
Where might it be written that a mistake is a crime?

@RobbieGoodwin it's not a crime, per se, but it turns out that it is against the regs, as per FAR 91.13.

Sorry Kenn; I just don't follow how a mistake can't be allowed… unless the suggestion is that this mistake is too serious to forgive. Is that it?

@RobbieGoodwin 757toga and Pondlife both give concrete examples of how it is against the regulations. Whether it's allowed or not is another matter.

“Allowed” versus choosing to prosecute are different things.  As I commented above, it’s a self punishing act, so why pile on?

What I meant is the the pilot suffers either way.  If your passenger is injured you will be responsible for medical bills.  Damage a rental, you cover the repair cost.  (Or your insurance covers things, and your rates skyrocket.). If an otherwise decent pilot makes a one time flub when stuff is hitting the fan inflight are you espousing that the FAA revoke their pilot certificate?

@KennSebesta Sorry; can you explain how what's "against the regulations" being "allowed" could be another matter?
If you insist that some mistakes are too serious to be forgiven, why not just say so?
Otherwise, why not accept that mistakes are… uh… mistakes?

@RobbieGoodwin there's an entire study of "insincere rules", that is to say actions against the regs but still permitted. For an example of a widely held "insincere rule" in the US,  see the description of automotive speeding on Page 509 of https://deliverypdf.ssrn.com/delivery.php?ID=331005097121007071076001077110103090117046093006002049011113031091078001124068028002000030020015104116044126067021099001117004041026000040040118083024090003097088051054085086092071023124072114071091020090076107079012030121065025120084093116020119003&EXT=pdf&INDEX=TRUE

Hey Kenn, why not ask your lawyer, or insurer, or local FAA office?

@RobbieGoodwin I'm not sure what further problem that would solve, since the authoritative answer was already given by 757toga. If you want to discuss this more, let's continue in chat, https://chat.stackexchange.com/?tab=site&host=aviation.stackexchange.com.

During my 5 years being involved with gliding, landing gear up was a relatively common occurrence, either because the pilot somehow forgot to extend it, the gear got stuck or wasn't locked properly in place before touchdown. As long as there was no damage to the aircraft, the aircraft was simply lifted up, checked over by an appointed club MX inspector and released to service again. No report needed filing. If however the aircraft sustained damage or the gear had not extended due to a mechanical failure, that would indeed trigger a report as well as a visit from an "NTSB" investigator. (although this was all EASA / club-regime)
In regards to the insurance, most aircraft insurances will cover damage caused by accidental wrongful operation of the aircraft provided that the pilot was training and checked out according to all standards. If however the occurrence is categorized as the product of gross negligence and carelessness from the pilot, that is an other story.
I can't count the amount of material damage to aircraft and vehicles that was caused in my gliding club because people mishandled material and or made simple mistakes, so as long as it is not intentional, the insurance and no-blame culture will usually have your back... although you premium may rise considerably if it keeps happening.

Another good way to do this is to forget to raise the gear on release, fly round all day wondering why your L/D is so poor, then move the gear handle when you enter the pattern.  You can't see the wheel, of course, and some gliders have the gear lever working in different directions.  My instructor had us prepare per-glider checklists with 'Gear lever FORWARD' rather than 'DOWN' for this reason.

The FAA has sanctioned pilots for landing gear up, as a violation of 91.13 (careless and reckless).
For example, a case from 2006: Administrator vs. Lorenz, NTSB Order No. EA-5205. There was only minor damage to the aircraft, and no damage to anyone else or anyone else's property. The pilot appealed to the NTSB, arguing that since there was no "potential endangerment" there was no violation. The NTSB upheld the FAA's position and the pilot lost.
Here are a few key points (the case has more details and references to other, previous cases):

In particular, respondent, who holds a commercial pilot’s license,
admits that on March 23, 2004, he inadvertently landed a Beechcraft
Model V-35B at the Eastern Iowa Airport with the landing gear
retracted. The aircraft sustained “minor” damage, and no other
property was damaged.

Landing an aircraft with the landing gear retracted clearly creates
the potential for injury, death or significant property damage, not
only to the aircraft and those aboard it, but to those persons or
property that are, or reasonably could have been, in the vicinity of
the runway.

Interestingly, the NTSB appears to say that who owns the aircraft is irrelevant, because even if it belonged to the pilot there's still "potential" danger to others who might be nearby:

We need not address respondent’s arguments regarding the aircraft’s
corporate ownership and whether it constitutes “property of another”
in light of our finding that his gear-up landing created potential
endangerment to other persons or property in the vicinity of the
accident.

Per an AOPA article, it's unusual for the FAA to pursue a 91.13 case for a gear-up landing:

While there are a few of these cases on the books, our experience has
shown that the FAA does not usually choose to take enforcement action
in these situations, choosing instead to handle the matter
administratively with a reexamination request or remedial training in
order to be sure the pilot has the proper procedures in place to
prevent a recurrence.

Perhaps in the case of Lorenz, there was more context or other factors that made the FAA bring the case. I couldn't find any more details about it.


text:
Why does true airspeed decrease with altitude?
I was having a discussion with a friend who's going for a record speed over distance attempt. We were discussing which altitude will have the fastest TAS, and I suggested it would be the highest altitude the plane can fly at.
I now realize this is incorrect, as shown by many charts such as this one:

The above chart shows for a give power output TAS increase linearly with altitude, but power decreases exponentially with altitude. What I don't understand is why.
As air density goes down, there is simultaneously less oxygen and less friction. I would have expected those to to act linearly and oppositely, i.e. TAS increases by as much as the percentage of oxygen-- and thus power output-- decreases.
However, clearly some other factor is at play, whereby the available power decreases faster than the density.
What is it? I'm thinking it might have something to do with inefficiencies in spinning blades-- propellers and turbines alike-- and pumping engines, but I don't have a convincing hypothesis which ties together observation and expectation.

Is it a turbojet, a turboprop or a propeller airplane?

The power that can be produced by a normally aspirated piston engine reduces with altitude -- as you note, the air gets thinner.  This is typically modeled as:
$\frac{HP}{HP_0}=1.132 \sigma - 0.132$
Where $\sigma=\frac{\rho}{\rho_{SL}}$ is the ratio of the density at altitude to the density at sea level.  Similarly, $HP_0$ is the power available at sea level.
When aircraft use forced induction, we usually use it to compensate for this lapse (not to boost sea level power).  This is called turbo normalization.  A turbo normalized aircraft will maintain sea level power up to some critical altitude and then the available power will begin to lapse.
The aerodynamic forces on the aircraft also vary with altitude.  Take lift for example...
$L=C_L q S$
Where $C_L$ is the lift coefficient, $q$ is the dynamic pressure, and $S$ is a reference area.
The dynamic pressure $q=0.5 \rho V^2$.  So, at fixed lift coefficient and velocity, the lift would drop off with altitude as $\rho$ drops.
Of course, lift must equal weight, so as altitude increases, we must either increase $C_L$ or increase speed $V$.
To fly at a constant $C_L$, we increase true air speed (TAS).  We call this constant equivalent airspeed (EAS).  Equal EAS will also be equal dynamic pressure ($q$).
Other aerodynamic forces behave similarly with altitude -- if we fly at constant $C_L$, then we also fly at approximately constant drag coefficient $C_D$ (ignoring Mach and Reynolds number effects).  So $D=C_D q S$ describes the drag force.
Since we need $L=W$ and if we maintain constant $q$ and constant $C_L$ with altitude, then we will also maintain constant drag coefficient $C_D$, drag force $D$, and even lift-to-drag ratio $L/D$ with altitude.  We will fly at constant EAS, but increasing TAS.
However, the power absorbed by the air due to drag is the product of drag and velocity (true) $P=D V$.  So, it takes increasing power to maintain flight at constant $C_L$ and constant EAS with altitude.
Unfortunately, as discussed previously, our normally aspirated piston engine's ability to produce power decreases with altitude.
Depending on how these factors balance, an aircraft's maximum speed may first increase with altitude and then decrease -- or it may be maximum at sea level and decrease with altitude.
Of course, we usually don't fly at maximum speed for extended periods of time.  Instead, we fly at a speed for best range (or slightly faster than that).  The speed for best range does not care about the lapse in available power (until we reach the altitude where we have insufficient power).
Because of this, the speed (TAS) that a piston prop aircraft operates at will usually increase with altitude.
Some of the above details are affected by changes in propeller efficiency -- and how that varies depends on whether you are operating a fixed pitch or constant speed propeller.  In either case, the above factors will determine a lot of what is going on.

Perfectly sensible. For a given power output, TAS goes up with the cubic root of density. And, IIUC, the subtle nuance at 100% throttle is that frequently as altitude goes up, necessitating an increasing AoA because EAS is slowing down, $Cl$ goes up sufficiently more quickly than $Cd$ such that the drag falls faster than the power output decreases. (Whew, that's a mouthful!) Did I get that right?


text:
Are flaps regulated as balanced control surfaces?
I was talking about flutter the other day and how it occurs with the ailerons able to flap out of sync with the wing. I realized that the flaps, could in theory, move in a similar way. However, the ideal flap is rigidly attached to the plane and has little to no movement.
Does the FAA/EASA consider standard flaps-- e.g. plain, slotted, fowler-- to be balanced control surfaces?

No flaps do not need to be mass balanced. They are too rigidly attached to be able to feed energy back into the main wing structure and get a self-amplifying flutter oscillation going.
To get flutter going, the surface needs a degree of freedom of movement both up and down (like an aileron), some flexibility (springiness) in its control input connection, a center of mass that is aft of the hinge axis so that vertical motions create rotating moments about the hinge, and finally, a bit of springiness in the main structure.
A non-powered aileron (operated by cables and/or rods connected directly to the control wheel) without mass balancing tends to have this problem, so mass balancing eliminates the ability of the surface to induce flutter, in spite of its flexible interface with the main wing, by eliminating rotating moments created when the surface is vibrated vertically.
For an aileron or elevator, you don't need mass balancing if the surface is connected to the parent structure with a rigid, non-compliant attachment. You can achieve this by driving the surface hydraulically, so that the only movement of the surface that is possible is that induced by the hydraulic actuators. When you see jets that have no balance horns or offset hinge lines on ailerons or elevators, it's safe to assume it has hydraulic controls.
A flap is similar to the hydraulically operated aileron, even if its attachment is mechanical. The operating mechanism creates that same rigid connection between surface and main wing so that it can't feed back energy into the wing, and thus no balancing of the surface is required.

Excellent points. Do you know if the FAA and EASA generally see it the same way from a regulatory perspective?

The OEM will have to demonstrate the design meets the regulatory flutter resistance requirements, so it would produce a model and analysis that would show the wing, nacelles flaps, ailerons etc are all protected against flutter. For something like a flap, analysis would show that the rigidity of the attachment does the job, and the experimental test pilots during dev/cert testing will find out if the analysis is good. The FAA has an Advisory Circular that talks about it. EASA would be similar. https://www.faa.gov/documentLibrary/media/Advisory_Circular/ac25-629omni.pdf

I supported a suspected flutter incident of an aileron on a CRJ after an in-flight vibration snag, and they then found the aileron had a lot of backlash (slop) from bearing wear (the actual vibration was coming from somewhere else as it turned out). The flutter analysis done by the flutter engineering group showed that even with all the slop, it was still flutter resistant b/c air loads kept the surface preloaded against the top of the backlash range, effectively maintaining the rigidity for flutter resistance purposes. Same thing with a flap even if there is a lot of backlash.


text:
Why does the typical magneto switch have R on the left and L on the right?
I can't think if I've ever seen this be done with the L on the left and theR on the right. Even on my Rotax 914, which is, *ahem*, modern and doesn't have magnetos, the key is still labeled with R on the left and L on the right.

Why?

I am certain that this has been asked before...

@MichaelHall I thought so, too, but hadn't found it. I'll give a more thorough look.

Possibly related question

This website gives a good explanation which I'll repeat here.

But… deep breath… selecting the right-hand key position grounds out the right mag, essentially turning it off, while leaving the left mag running — so the right key position is marked with an “L.” Likewise, moving the key to the left (but not all the way to OFF) now disables the left side, leaving the right side working — hence the “R” label at the left key position.
Key position dictates what’s “off,” while the label shows what’s on. So there is a certain logic to it, once you embrace it.

For a more detailed explanation see the website itself

But the "Both" and "Off" on OP's picture relate to what's on, right?

OFF means both mags are grounded out and won't work.  R means the left mag is grounded out and only the right one works. L means right mag is grounded out and only the left one works.

I read this as, "somebody crossed the wires, changed the labeling to match, and it became standard". Explains things, but doesn't make sense.

@FreeMan if I had to guess, it's because the rotary switch is the evolution of the fliip switches, http://www.avcanada.ca/forums2/viewtopic.php?p=841216&sid=f4d03657965028998c59e97ade4ff5aa#p841216. In those cases, flipping the right switch turns off the right mag. So it's a perfect mapping, although that doesn't mean it makes good sense, even if it makes sense.


text:
How (or where) do I find an airfoil optimized for low reynolds numbers (100,000~150,000)?
Looking to build a RC plane. Because the span is going to be small and the speed relatively low, I estimate the R-number to be around 100,000. Is there an airfoil optimized for that range of Reynold's Number?

Try the HQ series from the UIUC Airfoil Data Site. That is HQ as in Helmut Quabeck (model airfoils), not Horstmann-Quast (those would be glider airfoils).
Another possible source is the website of Martin Hepperle.
Unfortunately, there is not one, but hundreds of airfoils optimized for the model aircraft range of Reynolds numbers. To be any more specific, I would need to know more about how you plan to use that model:

Will it be a glider with a low wing loading, trying to stay aloft as long as possible?
Will it be a fast glider for ridge racing?
Will it be a powered model for aerobatics?

Each case will need different airfoils.

Right... I am thinking of a medium-slow speed (10-15m/s) powered plane mainly geared for endurance. I suppose it will be closer in design to a glider. Thanks!


text:
Specific Acrylics for fabricating windscreen
I've a experimental aircraft in need of a windshield due to crazing caused by fuel and cracking caused by stress. It is made of polycarbonate/Lexan and I want to replace it with acrylic because of its susceptibility to fuel and scratching. However, as there are a jillion kinds of acrylics I need recommendations of specific brands and types.

Are you sure you don't have that backward?  My experience with acrylic is that it scratches easily, cracks if bent just a little too far, and crazes from the slightest exposure to solvents (like gasoline, isopropanol, etc.).  Polycarbonate, on the other hand, is used for motorcycler helmets, windhshields, and other high-impact applications as well as being more impervious to solvents.

Have you tried contacting an OEM like LP Aeroplastics http://www.lpaero.com/ and see if they have one for your model of plane already?  Or at least browse their online catalog, link on their homepage.

Sorry but requests for product recommendations are off-topic pretty much everywhere on Stack Exchange.

You need cast acrylic sheet (not extruded).  If you are in North America you can get it from Aircraft Spruce https://www.aircraftspruce.com/catalog/mepages/plexiglas.php?clickkey=7342.  The cast sheet made for airplanes has UV blockers as well.
Acrylic has roughly the same tensile strength as Lexan (8-9ksi - a 1/8" sheet of acrylic or Lexan has roughly the same strength as a sheet of .032 6061 aluminum) but is more brittle and has to be formed into as close to the final shape as possible, with the minimum possible preload on the sheet and at fastener holes.
On the plus side of being brittle, although it still scratches a lot easier than glass, the surface is way harder and doesn't scratch if you look at with furrowed brow like Lexan does (although it can come with hard coatings - which tends to get removed by polishing), and is immune to fuel to boot.
Lexan is popular because it can take a lot of pre-stress and you can force it into moderate curves and clamp fasteners on it without it cracking.  But acrylic is superior for the other characteristics important to airplanes.
If your homebuilt's windshield is made up of flat sheets, it's a no brainer - cast acrylic.  If it's a curved wraparound, you'll have to form the acrylic into the curve by heating it in an oven and draping it over a form.  If fuel was an unavoidable problem because of a gas cap in front, it's probably worth going to the trouble to form an acrylic windshield if you have an oven big enough to fit the sheet.  If the fuel exposure was an unlikely to repeat fluke and you need a curved windshield, it might be easier to just do another polycarbonate one that you can cold bend.


text:
What are the radius of containment (Rc) and horizontal protection limit (HPL) and how are they related to Type Code?
In ICAO Document 9871 (Technical Provisions for Mode S Services and Extended Squitter) page 30 they have mentioned it in the form of a table. But they haven't explained HPL and Rc in detail.

I edited the question to clarify that the reference is to ICAO Doc 9871.  The question is clear and can remain open.

Mode S ADS-B messages use DF17 or DF18.  The type code field with the DF17/DF18 is set to reflect the current navigation performance.
The Containment Radius (Rc) is the radius that there is a 95% probability the aircraft is within that radius of its stated position, both horizontally and vertically.
HPL is computed by the GNSS and is the theoretical 100% containment (protection) limit in the horizontal only. It is approximately double the value of the Rc. Since HPL is horizontal only, the vertical is mostly addressed by the cases of 1) no altitude, 2) baro altitude or in its absence, 3) the GNSS height.
So based on the presence and value of HPL and altitude data, the appropriate Type Code can be set by referring to the table. The receiver of the message can then determine the transmitter's Rc.


text:
By what scale is Latitude and longitude calculated on aircrafts?
If we travel by 1 metre on any side how much of longitude and latitude changes?
Since the flight is travelling so fast latitude and longitude changes quickly so how do they calibrate it?
PS:Is there any formula related to this?

By "scale", do you mean the resolution (minimum discernable change between two measurements) or accuracy (typical deviation expected between measurement and reality)?

your first question (metre vs lat/long equivalency) is a geometry/GIS.SE question, so it is off-topic here.
your second question (calibration) need a bit refining: calibrate what? usually you calibrate instruments, and you don't do that in flight. what are you referring to here?
as for formulas, yes, there is plenty of formulas, which one are you interested in? (depending on which formula, it my be on topic or off topic here and you would be better asking on one of our sister sites)

If we travel by 1 metre on any side how much of longitude and latitude changes?

It depends where you start. One nautical mile is defined as a minute (1/60 of a degree) of longitude at the equator. This is about the same as a minute of latitude at the equator, but because of the shape of the earth, it's not the same as a minute of longitude anywhere else: lines of longitude get closer together as you head north or south, and they all meet at the poles. If the earth were spherical, the lines of latitude would all be the same distance apart, but it's not quite: it's squashed flat, like an orange, so latitude lines get closer together as well, but never meet.
Because it's simple geometry, you could make a formula to tell you the length of a minute of latitude at any latitude, and another to tell you the length of a minute of longitude at any latitude, but we don't do this, and here's why.
Latitude and longitude are a good way to describe where you are, but not how you're travelling. Instead, you plan to fly a certain distance in a certain direction. You can tell the distance by multiplying your speed by the elapsed time, same as any other navigation.
You can measure your position with reference to fixed points. In visual navigation (in sight of land), you can use geographic landmarks: a particular town is ten miles to the north, or you're directly over a lake. You can also use radio navigation aids: fixed radio beacons which tell you where you are in relation to them. These work even in bad weather or if you're too high to see the ground, and even airlines navigate this way. You can plan a route flying from one landmark or beacon to the next, until you reach your destination, without even thinking about latitude and longitude. When you need to report your position, even in an emergency, you do this by reference to these landmarks or beacons.
If you ever do need to know your latitude and longitude, you can do this by referring to your chart. You find the beacon or landmarks that you're at, mark your position on the chart with reference to those, and read off the latitude and longitude. No need for any formulae, and the only thing you have to calibrate is your compass.


text:
If the number of flights in the world exceed 2^24 how will ICAO allocate the address to the flights?
What is the approximate number of commercial flights in the world as of now?
If the number of flights do exceed will they use the concept of frequency reuse in cell phones(dynamic allocation of addresses)?

Flightradar tracks less than 2^14 flights right now. That means traffic volume needs to increase by a factor of 2^10 before you run out of addressing space.

I voted to close this "what if" question. This is purely opinion based and there is no clear way to interpret what ICAO will do, or what technology may be relevant in the distant future.

ICAO addresses have nothing to do with flights, the are assigned to transponders.

Addresses are assigned to suitable equipped aircraft, not to flights. The total of 16 777 214 available adresses is distributed in blocks that are assigned to individual states. Block size depends on the size of the states, e.g. the US and Russian Federation have block with 1048576 addresses, while e.g. Grenada or Mauritius only have blocks of 1024 addresses.
The allocation of addresses is decribed in ICAO Annex 10, Vol. III, Chapter 9.
An extract of the relevant part can be found here.
As long as the addresses are just assigned to conventional Mode-S transponder equipped aircraft there should be sufficient free addresses. This could change if all airspace users would be assigned a unique address, e.g all drones participating in future UTM (urban or unmanned traffic management).


text:
What is the air/fuel mixture ratio in a turbojet engine?
How does it relates to stoichiometry ? What are the possible extinction accidents related to air/fuel mixture ratio ?

Do you have a specific engine in mind?  In general it can be pretty broad.  There are both lean burn (excess oxygen) and rich burn (excess fuel) combustors out there.  Even for a specific engine the air/fuel ratio can vary depending on operating condition.

@DanielKiracofe The engine I have in mind is the SNECMA Turbomeca Larzac 04-C6
https://en.wikipedia.org/wiki/SNECMA_Turbomeca_Larzac

This cannot be answered specifically, the stoichiometric fuel/air ratio (FAR) is based on burning all oxygen, while for cooling purposes the airflow is generally much higher; therefore the FAR will be lower than the stoichiometric FAR. Note that you do not want to burn fuel at stoichiometric conditions as this produces the most nitrogen oxides. Fuel is being burnt rich and then quickly burnt lean.
Note that the FAR is depending on the speed, altitude and power setting (and the type of engine). To quantify the FAR, you would need to model the engine and simulate the performance. As an example (very simple to show the effect) a turbine exit temperature sweep for different altitudes and Mach numbers is shown below for a simple turbojet engine (the engine linked in the comments of the question is practically a turbojet engine considering the very low bypass ratio). You see that the FAR is variable (top graph is the burner exit FAR, bottom is the stoichiometric FAR):

I cannot address the second part of the question regarding the accident rate, that does not seem to be relevant.


text:
Airplane data for thriller novel
I've written a legal thriller. One scene involves the antagonist lawyers hiring someone at O'Hare to sabotage some other lawyers' private jet so that it crashes. But I don't know enough about aviation to write a plausible scene, as in what could someone on the ground do to down a plane that the pilots wouldn't see. So far, I've heard about someone deliberately installing the wrong size screws in the windshield because the pilot could miss them in pre-flight checks. This could cause the windshield to fail at altitude, either crashing the plane or ending in an emergency landing at the closest field. But are the screws are visible from the cockpit? And if so, would the pilot detect a problem before takeoff? Any guidance would be great. Thanks!

Does this have to look like a accident, especially afterwards? Something which looks like a vibration fatigue failure maybe work in this case, but  is probably fatal and require some kind of access to plane and have to look like maintenance. If you don't need to hide you can manipulate the fuel in one of the tanks, can be done quickly, which will only be used later during flight (depends on plane type?) and cause all engines to fail. Here is the chance much higher for the pilots to save the situation. Of course your free to serve the pilots decaffeinated coffee, serious harm ;)

I would suggest to look into Wikipedia overview on Category:Airliner accidents and incidents caused by maintenance errors, immediately focusing on the "Investigation" sections. Some errors could be reproduced deliberately.

Depends on the plane. On many the windshields are sandwiched in place between riveted strips of metal - no screws.
Might have better luck messing with the O2 system, reference the golfer Payne Stewart's jet plane accident.
From Wikipedia:

On October 25, 1999, a month after the American team rallied to win the Ryder Cup and four months after his U.S. Open victory, Stewart was killed in the crash of a Learjet flying from his home in Orlando, Florida, to Texas for the year-ending tournament, The Tour Championship, held at Champions Golf Club in Houston. National Transportation Safety Board (NTSB) investigators concluded that the aircraft failed to pressurize and that all on board were incapacitated due to hypoxia as the aircraft passed to the west of Gainesville, Florida. The aircraft continued flying on autopilot until it ran out of fuel and crashed into a field near Mina, South Dakota.


text:
If the VOR works and the TACAN works, why doesn’t the VORTAC work?
Here https://www.tailstrike.com/270680.htm is the transcript of the CVR of the Itavia Flight 870 (Ustica Massacre), but some communications have been omitted; I’ll use this site: http://www.noidellitavia.it/176334379.
I read (I’m translating):
20.23.38 =ROMA= 870 what’s your distance from Firenze? [FRZ was a VORTAC, now is the LOMED point]
=IH870= Approaching Peretola, 15 miles south [from FRZ].
=IH870= 870 is exactly aligned with the Firenze radial, heading 153 [FRZ – Bolsena (Amber 14), BOL is now the GIKIN point].
It’s evident that both the VOR and the TACAN are working. But:
=IH870= We have to change our mind about the Firenze VOR functionality
=ROMA= Yes, it's actually not going very well.

IH870 receives bot the VOR and the TACAN, but it complains about the VOR functionality. What happened?
Is it possible that the VOR is malfunctioning, while the TACAN works like a charm?

TACAN is a military technology not used by airliners, so the flight wasn't using TACAN to begin with.

@GdD How do he know "15 miles south"? The TACAN is used as DME.

@GdD It is normal for civilian aircraft to use the DME part of TACANs

That wasn't clear win the question @Cristiano. DME is only one part of TACAN, you may want to be more specific.

What should I clarify, that you don't know what a TACAN is?

VOR, TACAN, and DME are three entirely different pieces of equipment. The fact that they're all located at the same place and referred to by the same identifier doesn't change that. There's no such thing as "the DME part of TACANs", they're two entirely different things. So, yes, it's entirely possible for the VOR part to fail while the DME still works or vice versa. Is that what you were asking?

@HiddenWindshield Yes, thank you. I would be interested in knowing how a VOR fails: could it give wrong radial readings? In the sense that the airplane HSI/RMI shows 153 while the true radial is 142.

Why is this downvoted?

DME is a subset of TACAN. DME transponders and interrogators are abbreviated TACAN transponders and interrogators. Civilian aircraft only use DME but that just means they ignore the rest of the TACAN ground station broadcast when they interrogate a TACAN ground station. A ground station will never have both DME and TACAN transponders: if it has TACAN, DME is redundant. It's possible for VOR to malfunction while a co-located TACAN or DME still works. and vice-versa.

+1 for the question #2. Any idea for the question #1? I mean: is it possible that the onboard instruments show a wrong radial?

I thought I answered question #1 with my last sentence. Wrong radial, as seen by the aircraft is possible. It would depend on what went wrong with the ground station. Unless it was way off I don't know how the aircraft would notice. I think in this specific case it stopped working altogether.

"Wrong radial, as seen by the aircraft is possible. It would depend on what went wrong with the ground station". The VOR integrity is monitored on the ground: "The monitor shall transmit a warning [...], and either remove the identification and navigation components from the  carrier  or  cause  radiation  to  cease  if  any  one  or  a  combination  of  the  following  deviations...", and for bearing the deviation is 1° (though some national AIP require less than 1°). (ICAO Annex 10.) Perhaps in 1980 things were different.

Very interesting, thank you, but I'm not able to find your citation. Would you be so kind to tell me the exact volume and page?


text:
Why, on a short-field takeoff, are we taught to run the engine to full power before releasing the brakes?
TLDR; See Update #2 and #3. The FAA on the one hand says "It has not been established that this procedure results in a shorter takeoff run... " but then requires the same procedure as part of the check-ride. Interesting!

The standard training for a light single-engine short-field takeoff is to run the engine to full throttle before releasing the brakes. Why? This seems hard on the engine, and the 3-5m of ground roll it potentially saves is utterly irrelevant for any kind of sane flying.
Is it just that when doing a short-field takeoff, the regulators would like us to know, and employ, every arrow in the quiver of maximizing takeoff performance?
Or, put another way, once I take all other factors into account, and leave myself a sane factor of safety (which is going to be several hundred meters, not 5 or 10), is there a subtle reason why running up the engine and checking the instruments before releasing the brakes is any different from checking the instruments during my ground roll? In both cases my life is on the line and I am extremely self-interested in doing a complete and thorough job.
And, put one last way, why shouldn't we just do short-field takeoff procedures on every takeoff? If there's no harm to the plane, what do we stand to lose?

I believe the best answer is https://aviation.stackexchange.com/a/94370/20394. Ushby eloquently argues that the problem is not having 15' extra room taking off, it's having 15' extra stopping room in the event of an aborted take-off.
Note, however, there is still room for an explanation of why standard procedure for all takeoffs is not to run the engine to full-power before releasing the brakes.
Update #2
https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/airplane_handbook/07_afh_ch6.pdf

The pilot should apply takeoff power smoothly and continuously, without hesitation, to accelerate the airplane as rapidly as possible. Some pilots prefer to hold the brakes until the maximum obtainable engine revolutions per minute (rpm) are achieved before allowing the airplane to begin its takeoff run. However, it has not been established that this procedure results in a shorter takeoff run in all light, single-engine airplanes.

Update #3
We learned from 757Toga that the aviator's handbook and the check-ride requirements are inconsistent:

Doing some additional research, it seems that the Private and Commercial ACS (PVT ACS https://www.faa.gov/training_testing/testing/acs/media/private_airplane_acs_change_1.pdf) require full power, then release the brakes as part of the checkride requirements to demonstrate a Short-Field Takeoff for a Commercial or Private Certificate. In the Private ACS it's Skills PA.IV.E.S7/8. (page 25 in the PVT ACS.)

Comments are not for extended discussion; this conversation has been moved to chat.

My instructor also taught me that the full-power run is when you lean for best power at high density altitude.

In a short field takeoff things happen fast, the window for an abort is small, so the main reason for running the engine to full power is to make sure full power is available before you start to roll. It takes a few seconds to develop full power, and you don't want to be a quarter of the way down the runway before you find out it's running rough or isn't at full RPM - by the time you realize there's a problem you may be past the point where you can safely abort your roll.
Going to full power before you start your roll lets you give the engine instruments your full attention, you don't just push it all the way forward and then release the brakes, you should check your RPM and then run through the critical instruments: temperature, pressure, charge and suction. Once happy with those you then release your brakes and concentrate on flying without distractions.
Also, it may be only 30 feet or so, but 30 feet can make a difference in a short field takeoff. Anything you can do to get that little bit more is worth it.

Comments are not for extended discussion; this conversation has been moved to chat.

Two reasons come to mind. First, you want maximum acceleration to minimized distance to reach takeoff speed and this provides that from the very start. Second, the increased airflow from the propeller provides increased authority to the elevator, allowing more pitch-up if doing this on a soft field (where short field performance often happens), thereby unloading the landing gear and its surface drag. If flaps are down on a high wing aircraft, some of the prop wash may strike them, providing lift even at zero ground speed.

״providing lift even at zero ground speed״ Lift is never dependent on ground speed.

@DeepSpace While technically correct you ignore his point that additional lift can be created without ground roll, on which a short field takeoff does depend.

@DeepSpace but lift is dependent on air speed, and air speed is dependent on ground speed

@Christian Is it? You can have a ground speed of 0 and an airspeed of 100 knots in a very strong head wind. You can even have a negative ground speed and a positive airspeed if your brakes are not very strong :)

@DeepSpace, “Lift is never dependent on ground speed.”  That’s Jim’s point…

@Pilothead: Is lift from your own prop wash negligible or not?

@Joshua The amount of lift depends on the amount and deflection of the propwash, so more is better. The Ryan 92 VZ-3 could lift itself off the ground with two huge props and giant flaps, so not even close to negligible. The NASA X57 uses eighteen electric motors spread across the wing leading edge to effectively double lift, also not negligible. A stock c150 is probably closer to negligible, though still non-zero.

I agree with all those points, but for the purposes of any sane short field takeoff in a small plane they're practically indistinguishable from a normal takeoff. Once I go to full power I have all the benefits you describe, and I hit full power in almost the same distance. In a Citabria, which has pretty decent short field performance, the difference between full power before releasing brakes and full power in a normal takeoff is around 1% of the absolute minimum distance to clear a 50' obstacle. That seems so tiny as to be irrelevant except in extreme cases.

@DeepSpace than why do we need runways at all? just take off from a standstill if groundspeed doesn't matter

@Christian because we don't have huge fans that can generate ~170 knots headwind at will deployed at every airport.

@DeepSpace well... why don't we? :)

As a general rule you are correct that the AFH does not require a pilot to run up to maximum power while holding the brakes, prior to starting the takeoff roll.  That being said, many airplane POHs (eg C172) describe this as the preferred technique in the Normal Procedures section.  Where an AFM lists this technique, it will also be the takeoff technique used for charting the ground roll and obstacle clearance distances in the Performance section of the POH.  Deviating from this, or any other technique published in the POH, does not guarantee you that your aircraft performance will matched the published data.  And on a real short field takeoff, a few extra feet might make the difference between success and tragedy.
The real advantage in holding the brakes and running full power prior to beginning to take off role is, when the brakes are released, it guarantees you the aircraft engine is operating at full available power throughout the entirety of the takeoff roll.  Engines often takes several seconds to reach maximum power, and that can eat up valuable runway.
For light airplanes, a brief run up to maximum power prior to beginning to take off role while holding the brakes will not harm the aircraft. Excessively running the engine at maximum power with minimal air speed could potentially pose an overheating problem, but it would take several minutes of doing this continuously to begin to pose such a risk.
In short, the AFH is correct as a general technique, but the pilot should always operate the aircraft using the short field technique specified in their airplanes flight manual.

That's a solid point about the POH, the one for the 7ECA says the same. Although if the FAA states that there is no proven difference for light singles-- which the 7ECA and C172 definitely are-- then I have to wonder where the manufacturers' recommendations came from. Do they know something the FAA doesn't? Or were they were meeting the FAA's past expectations, which have now updated?

In truth, I’m not sure why the FAA even makes that comment in the airplane flying handbook, as I have never heard of a manufacturer of a light single engine airplane using another technique for short field takeoff.  Larger, more powerful, conventional landing gear aircraft such as a P-51 cannot follow that technique as the tailplane cannot maintain pitch authority with the brakes held above 40” MAP.  But every light, tricycle gear airplane over heard of - Cessnas, Cirruses, Diamonds, Mooney, etc. - holds brakes w/ a full power run up.

It has no effect on the engine whatsoever.
Aero engines are highly decoupled from the drivetrain via a very hefty fluid coupling.
Now in engine design, insulating an engine from drivetrain vibration (think: tracked vehicle) is very effectively dealt with by a fluid coupling.  And aviation uses a compressible fluid -- the air.
The engine/propellor has no earthly idea that you're holding the brakes and that the plane is not moving.  Its frame of reality is only airspeed, which is subject to winds and varies, obviously.
Thus, the matter of "holding brakes" is entirely a human perception, and it is not a reality that affects the engine.  Holding the brakes in a 10kt headwind is no different than rolling 7kt in a 3kt headwind.
While the vibration may be highest at lowest speeds, keep in mind that aircraft engines and mounts must be designed or much worse than that - they must be designed to endure a broken prop.  While it's true aluminum has no fatigue limit... an aircraft which was degraded by run-up vibration would be unworthy of a certificate.

aircraft engines are not "highly decoupled from the drive train" - quite the opposite

@warren Think more broadly. they are highly decoupled by nature, since they are pushing on air. The air is compressible and provides far superior decoupling to a liquid torque converter.


text:
What is the legal definition of "transient" when referring to airport closure?
As of a few days ago, 1B6 is now CLSD TO TRANSIENT
This is the NOTAM which appears at https://notams.aim.faa.gov/notamSearch/nsapp.html?METHOD=DISPLAYBYICAOS&REPORTTYPE=RAW&FORMATTYPE=DOMESTIC&RETRIEVELOCID=1B6&ACTIONTYPE=NOTAMRETRIEVALBYICAOS#/details
!BDR 07/063 1B6 AD AP CLSD TO TRANSIENT 2207121715-PERM
I think the meaning is relatively clear-- don't land here unless you're based at this runway-- but if a pilot is held to a standard of obeying the NOTAM it would seem there should be an iron-clad definition of TRANSIENT.
Is there a legal definition of TRANSIENT in the NOTAMs?

I get where you're coming from, but I feel like that's a very unsatisfactory legal standard. The NOTAM isn't even written in plain English, there are many ways to willfully come to another conclusion than the one I did, and they would be defensible.

On the Air Traffic side, we use "transient" to mean any aircraft which lands to a full stop or departs and exits the local area—distinguishing it from a "pattern work" aircraft. This is likely not the meaning intended by the NOTAM.

Not to be flippant, but I’d be more curious about the definition of “closed”. What if you don’t need fuel and plan to park at a friend’s hangar? Because if the airport isn’t private then it’s public, and if it’s open, then it’s open to the public…  Something like “transient services not available” is more meaningful. (And typical)

In the particular case of 1B6, airnav.com lists it as private so I guess they can do what they want. I also called them because I was curious, and it turns out their runway is closed for repairs for a little bit, which is what motivated the NOTAM.

It seems that transient may be defined by local authorities or state authorities for example the city of Livermore CA defines it here:

“Transient aircraft” means any aircraft which utilizes the airport for
occasional temporary purposes, generally no longer than seven days,
and which is based at another airport and is not assigned a reserved
tie-down or hangar at the airport. (Ord. 2085 § 1(A), 2019; Ord. 2065
§ 1(A), 2018; Ord. 1916 § 1(5), 2010; Ord. 1905 § 1, 2010)

While Mansfield Municipal Airports Master Plan has it defined similarly:

Transient Aircraft –refers to an airplane whose home is at an airport
other than the airport for which the forecast is being produced. In
other words, any aircraft that uses the Airport, but whose home base
is at another airport, is a transient aircraft.

I can not find a definition directly for Hopedale Airport but its a small town and they dont seem to have that much digitized. MassDOT also does not have an official definition that I have yet found. Interestingly enough the Order which defines NOTAMS uses CLSD TO TRANSIENT as an example but does not define it anywhere.

Thanks for the excellent answer. Although the thought of digging up municipal documents in order to fully parse a NOTAM is horrifying!


text:
In what way is a Rotax 914 F2/S1 different from a standard 914 F2?
Rotax manufactures the 914 F2/S1 variant, which is (at least) used in Stemme's S10 and S12 self-launching gliders.
What are the differences between the F2 and the F2/S1?
Bonus question: Why have any differences at all?

The Stemme S12 maintenance manual discusses this:

The engine of the STEMME S12 is based on the ROTAX 914 F2-01, which has been certificated by
BRP-ROTAX® GmbH & Co.KG, Austria. STEMME modified the arrangement of some accessories
(induction and exhaust system including turbo-charger, engine mounts etc.) to adapt the systems
to specific installation requirements of the S12. These modifications are certified in the STEMME
S12 as engine model ROTAX 914 F2-01/S1.

and later in the manual:

The ROTAX 914 F2/S1 is modified by STEMME and based on the type ROTAX 914 F2. The modification
was completed as a co-operation with ROTAX and with approval by ROTAX.

...

Due to special requirements for installation in the center fuselage, the following modifications were made:

Relocation of the turbocharger unit to the aft of the engine to stay within the outlines of the S12 fuselage. The turbocharger unit is supported by five struts aft of the engine. A supercharger inter-cooler is installed. Relocation of the turbocharger unit required modifications of the exhaust system. The exhaust headers are attached to the turbocharger by springs, positioning them in spite of the high thermal stress.

The layout of the oil pipes for the turbocharger are modified.

The exhaust headers and the muffler are shrouded by temperature-resistant material, thus thermally isolating the system from the engine bay. The turbocharger unit and the air box are isolated by radiation protective shields.

The original ROTAX engine mounting is not used. STEMME specially developed an engine mounting for a center installation, consisting of two upper and one lower supporting element.

In the liquid cooling system, the combined function of the expansion reservoir and refill container was split up in two separate containers, with the expansion reservoir located above the engine and the refill container on the left side of the fire-wall.

The ignition unit is installed above of the engine slightly behind the original ROTAX-position.

The throttle levers on the carburetors have been slightly modified (modification does not affect the throttle rigging of the original engine) and additional springs have been installed (pulling towards full-power position) to compensate for friction due to the long control cables between cockpit and engine bay.

So, the placement and mounts appear to be different, and the major reason is (presumably) for certification of a narrower fuselage.

This is great! I took the liberty of copy-pasting the exhaustive list of modifications, in case sometime in the future the link is no longer live. If you don't like the changes, feel free to revert to your original form.

@kennsebesta totally fine. I was trying to decide how much to paste. I thought I'd only be able to find the first paragraph but the list is very detailed.


text:
What U-2 design challenges are still challenging?
The Lockheed U-2 was first introduced in the mid-1950s, close to the dawn of the jet age. It seems reasonable to think that the technology required to design and build it then is rudimentary by modern standards, and, as a result, has become far more accessible. And, yet, the U-2 stands in a class on its own, with few, if any, equivalents in performance.
Let's say that I wanted to make my own U-2. Are there design challenges that would still be hard to handle even 70 years on? Or is it more that the requirements of human flight to 80K' are so expensive to meet, that, when compared to the price of a satellite, it's not worth the time?

I think physics and human body haven't changed in the past few decades  So the same limitations still apply

While operating slighter lower we have the privately built globalflyer https://en.wikipedia.org/wiki/Virgin_Atlantic_GlobalFlyer and there are various new build drones that routinely operate in the U2 environment https://en.wikipedia.org/wiki/Northrop_Grumman_RQ-4_Global_Hawk.

@sophit physics might not have changed, but materials science and manufacturing abilities definitely have improved, so we might see a different outcome because we can do more with less today.  So no, the same limitations dont apply

I think the biggest challenge at the time was the engine only makes 250kg of thrust at high altitude. If you would reinvent u-2 with newer engines then it becomes SR-71 and mig-25.

@user3528438 I can't agree with that. The SR-71 and MIG-25 are planes which were designed to push the limits on how fast a plane could fly. The U2's job was just to go high and loiter. It's much harder to go Mach 3 than it is to hold the plane steady at 95kts.

@Moo : yet even with better materials and science, it would still be extremely expensive and difficult. Not as difficult as back then, but still difficult, so if the same missions can be performed by other means (and the existing U2 can perform them still), it's not worth the investment.

@Moo materials and are knowledge are tools available to solve a problem. materials only offer a limitation in the sense that there's nothing more that can be done, physics and the human body give the limitations in the sense that there's nothing more that should be done.

@Federico - that's very deep :)

Read the "Design" paragraph of https://en.wikipedia.org/wiki/Lockheed_U-2

@EdRandall, thanks, anything in particular there I should be on the lookout for? I glanced at it and it sounds like lots of era-specific problems. Hard for a human to fly is trivial for a computer, and 1950s pressurization technology is rudimentary by today's standards.

@KennSebesta I was thinking particularly of two things, the Coffin Corner where stall speed and Vne converge, a computer can help but not completely fix that.   Also the ground effect issues with high-lift wing on landing though perhaps that is solvable.

@EdRandall I feel like my autopilot code could absolutely nail a 5kt window. Although I don't know much about high altitude flight so maybe it's significantly harder than I realize to pitch up/down when at 80k'. For landing gliders, in general we want/need air brakes to set it down on the ground. They're amazingly effective.

The really big challenge on the U2, which is actually based on a F-104 Starfighter fuselage as a foundation, was creating an air breathing engine that would run at 70000 ft.  P&W produced a modified J-57 engine (J-75 on later airplanes) with near zero compressor blade tip clearance to get it to run at that altitude (that is, the blade tips would initially brush the outer wall of the compressor case and there would be a kind of "break-in" to wear off just enough to make a tiny clearance, way less than the regular engine).
The other issues were mainly stability and control challenges related to the extremely tight operating envelope, than can easily be dealt with via software today.
It has an L/D of 23:1, making it kind of a heavy, oversize powered version of a Schweitzer 1-26.  It'd need a mighty big and strong thermal to be able to climb unpowered though.

"is actually a heavily modified F-104..." at first I laughed, then I scratched my head and did a little looking...  There's some truth in this, but I'm not sure they actually took a 104 and "modified" it, more likely they took lessons learned and blended some design elements into developing the U2.  Maybe clarify?

I was exaggerating a little bit for dramatic effect, but it did start out with an F-104 fuselage as the foundation you might say. If you look at an A-model, cover up the fuselage aft of the intakes, you'd think it  was a 104.

Ah... TIP clearance.  Another edit to make.  Near zero tip clearance is common now, but in the mid 50s it was a radical thing P&W proposed to Lockheed to get the J-57 to go that high without flaming out.

Is it right to assume that tighter compressor blade tip clearances would make for fewer losses due to shear flow between compressed and uncompressed air, as well as slightly more usable air overall? Or is there more to it?

Wouldn't the debris broken off by the compressor blade tips in their first runs, cause damage? Or are we talking about slivers of material so small as to make that a non-issue?

@Jules Yes the idea was to get every last ounce of efficiency from the blade.

@IanKemp IIRC the blades would just make brushing contact and would wear off enough material until there was no contact.  This was radical at the time, but modern engines are made with extremely small tip clearances. The CF 34 had a core lock problem due to this, where the core could jam from blade contact following a high power flameout due to shrinkage in the case. It became a normal production test procedure to stop the core in flight, and if it couldn't be got windmilling again by a certain diving airspeed, there was a "break-in" procedure to follow to wear the blade tips down.

Fascinating answer, thanks. So I just have to find a donor 1-26 on barnstormers, hook up a few dozen close-tolerance model airplane ducted fans, and wheeeeee!

@KennSebesta well if you look on youtube there's a vid of a 1:26 with a Jet Cat (I believe, or something similar) micro turbojet on it. In Canada a guy mounted a 2 stroke on a fixed pylon on a 1-26.

What U2 design challenges are still challenging?

I think that most of the technological limitations of that era would be today quite a bit less restrictive, basically in all fields of competencies.
Off the top of my head, modern technologies would bring the following improvements if available back then.

propulsion $\rightarrow$ modern engine would supply a better specific fuel consumption i.e. more thrust for less fuel; this would give more weight and space for the payload and/or more range and height;

structure $\rightarrow$ using carbon fiber reinforced plastic wherever possible instead of aluminium would give some 15% lighter structure, also improving payload and/or range;

aerodynamic $\rightarrow$ airfoils could be better optimised; for example using some supercritical airfoil, less drag at transonic speed could be expected, enlarging the coffin corner toward higher speeds making the flight envelope a bit less sensitive to speed changes;

avionics $\rightarrow$ well, who needs a pilot anymore?

Are there design challenges which would still be hard to handle

If we really need a pilot in the loop, I think that this would still be the only limiting factor today as it was back in the 1950s, because its presence would entail the need for a cockpit, a seat, a pressurisation system, windshield, suicide pill, ... all things using up space and weight.

Given the excessive regulations and safety precautions of today, much of those gains would be eaten up. +1.

Well, as said the human factor is the limiting force still in action  Thanks for upvoting

lets say I wanted to make my own U-2

The advantage these days is real-time transmission of data rather than bringing film back and developing it.
Back in the 1950s it was essential to get the results into the hands of decision-makers as soon as possible.  ICBMs and submarine launched BMs made this situation even worse.
So, modern, low budget "U-2" would be ... a balloon, possibly solar powered with propellers mounted in a duct to avoid detection.  If this was made in a "stealthy" form, it could be virtually invisible.  No need to fly near Mach speed leaving a hot jet trail behind.
The U-2's advantage was that its height originally made it impervious to interception even though it was radar detectable.  Once missiles were able to accurately reach that height it was time to move on (to the SR-71 and satellites).
Modern defenders need to find ways to watch out for these balloons.  A clever military tactic would allow a few "easy" ones to be caught, while many others remain undetected.

I think NORAD is already "watch[ing] out for these balloons"?

I have a hard time believing that the recent spy balloon/s really could maneuver other than possibly some limited ability to release ballast and helium/hydrogen (whichever it was) to navigate vertically.  If they were meant to move horizontally at a speed other than the wind speed, they would be shaped like blimps!

Good answer WRT the relevance of the U2 from a mission standpoint.  Not sure that’s what the OP was thinking though…

@MichaelHall Total pipe dream is to build my own U-2 so I can get to 80k' and see what the world looks like from there. But that got me to thinking about why no one else has built what is otherwise a very simple machine: a self-launching glider with a massive jet engine.

@KennSebesta, that is a pretty lofty dream!  (Pun intended). Complexity will come from other than power and aerodynamics though:  Pressurization, oxygen, IFR certification, etc.

@MichaelHall Haha, nice! TBH, I don't think there's much complexity aside from power. There's tons of surplus hardware out there for life support, and, throwing a pun right back at you, you're not going to get on anyone's radar for asking about pressure suits. There's not even IFR airspace above 65k'. Hard part, AFAICT, is getting above 40,000'

@KennSebesta, but you'll need an IFR clearance to transit the Class A airspace 18K to 60K.  I admit I haven't browsed for pressure suits on ebay, so maybe that part is surmountable

The major issue with the U2 / TR1 is that at altitude the stall speed and sound barrier are nearly the same. With the thin air there is less lift, requiring the large sailplane like wings. At the same time the speed of sound decreases with the lower air pressure, but it is still dense enough to create a shockwave if the speed of sound is exceeded.
If the wing stalls, you normally drop the nose to gain more airspeed, but this can cause you to break the sound barrier and the resulting shockwave will damage the wings, possibly severely.  Flying the U2 / TR1 was always a balancing act to keep above stall speed but below Mach 1.
Another fun part was trying to land. At low altitudes the wings generated a lot of lift even at low speeds. You basically had to fly it on to the runway and hold it down until it stopped moving. You did this while it balanced on one wheel. The wheels out on the wings fall off during takeoff as the wings don't have room to retract them. Early designs had two trucks with mechanics riding in the back trying to reattach the wheel as the plane was flying down the runway.
Please note I am not a pilot; I was in aircraft maintenance, but I worked B52 and KC135 aircraft. I knew people who had worked on the U2 / TR1, but I never did so myself.

There may have been attempts to catch the wingtip before the plane stopped moving after the main wheel was on the ground, but I can't believe they were actually trying to re-attach the tip wheels at any point when the plane was in motion, to say nothing of when the plane was still in the air!

Do you know how wide the U-2's "coffin corner" is at altitude?

The range between stall speed and Mach 1, the coffin corner, eventually becomes 0 at some altitude above 75,000 feet.

There was reason they stopped trying to reattach the wheels, the wings were modified to add a skid plate. once the plane slowed down enough one of the wings tips would simply start dragging on the ground, The plane would then shutdown engines and get towed off of the runway.

"the speed of sound decreases with the lower air pressure" Speed of sound for gases depends only on temperature and molecular structure, not on pressure.

For an ideal gas, pressure does not affect the speed of sound, but temperature does. For the earth's atmosphere, the temperature will decrease with altitude up to the stratosphere. Above the stratosphere the temperature will actually start to increase, but at those altitudes we tend to not use wings, but rely on pure thrust instead. With aircraft instead of relying on temperature to indicate altitude, we use air pressure instead.

You're implicitly saying that the "coffin corner" is still a problem for modern designers, that we couldn't design a trans-sonic wing that could handle Mach 1 at least during stall recovery?  (Without giving up the lift/drag and light weight of the U2's actual wing design?)  I guess that would require a swept wing, and that's hard for a high aspect ratio wing without bracing. (like Boeing is experimenting with for future airliners: https://www.youtube.com/watch?v=zkrdIw8LWrM)

Using today's technology, you could [re]design the U-2 using more "stealth" technology as well as using a more traditional tricycle undercarriage.  Everything else on the aircraft is fine the way it is!  Of course the use of more carbon fiber components for further weight reduction would be a good idea as well. For every pound of weight removed is a pound of weight of more sensors, or higher in altitude you could climb. Physiologically speaking the pilot would still need a Full-Pressure-Suit, in the event of a cabin depressurization.  Do away with the pilot you say?  Ask Northrop-Grumman how their RQ-4 Drone (that was supposed to replace the U-2) worked out?
There is a limit to the altitude you can climb while using a traditional "Air-Breathing" turbofan engine.  The higher you climb the less air(oxygen) molecules there are to support combustion, unless you go super/hyper-sonic and use a RAM/SCRAM-jet engine, but that uses A LOT of fuel. Traditional Jet engines can only consume subsonic air (No shockwaves allowed in the inlet). To fly higher you must either add more wing [area] (for subsonic flight) or fly faster!
Any aircraft that is optimized for high-altitude(thin-air) flight will always be a "pig" to land in the thick air.  The U-2 isn't so difficult to fly, it is VERY difficult to land! More Wing Area for higher altitude flight would make it a bigger "pig" to operate/land in the thicker air. The U-2 was designed to "Loiter" for many hours on end and flying supersonic uses up all the fuel too quickly (Ask a SR-71 driver).
So, your DIY U-2 needs to be designed to be more stealthy, using lighter/more-exotic materials, with a slightly larger wing [area] (for higher flight) and tricycle gear. The orignal prototype took 18 months to deliver to the CIA for a cost of \$ 1-million USD  (\$11,163,059.70 USD today).  So, your biggest challenge today is designing and building one for \$11,163,059.70 in today's money!  Not gonna happen, so just keep using the ones that are already paid for with 1986 money (those are the most current U-2S').
PS...U-2s rarely ever carry a "wet-film" framing camera anymore.  They are up to "modern" standards in avionics/navigation and sensors.


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What is the red barb for in the NOAA weather briefing?
Wind barbs are well-defined symbols, but I noticed in my WX briefing that there are now occasional red barbs in the chart.
The red appears on both the 5kt and 10kt barbs, and sometimes for largest 10kt barb or the smallest 5kt barb.

When I look very closely, I see that the red is actually an overlay, which hints that it might be related to gusts, but it's unclear how to interpret that overlay.

what about the red line underlying the direction? See the symbol in the center, east of Chicago

I'm pretty sure that's the wind overlay poking through. If you zoom in you can see it on all the wind barbs which have gust information. The degree to which it is evident is a function of the barb's angle.

@Ron Beyer gives a great answer. The red is not an overlay on the black. But, it does indicate gusts. The body of the wind barb indicates the direction from which the wind is coming. Each fletching-like line at the end of the barb indicates 10 knots of wind. A half a line indicates 5 knots of wind. If the line is black, it indicates sustained winds. If the line is red, it indicates gusts.

@EarlGrey - The red lines do not indicate direction. They indicate speed, only. The symbol you mentioned East of Chicago is two symbols. One is for a weather station on Lake Michigan. The other is for a weather station in the state of Michigan. They are just very close together.

They do represent gusts:
From AviationWeather.gov:

Figure 1: Station Model Plot
In many cases, the data are plotted using standard symbols and
plotting models. Fig. 1 shows the station model used in the METAR and
TAF plots.

Ceil - Ceiling height in hundreds of feet. This represents the height of the lowest broken or overcast cloud layer.
Id - The 4 letter ICAO identifier for the site. KRFD is Rockford IL.
Windbarb - The wind barb. The stick points in the direction the winds are coming from. Each full barb represents 10 knots of wind.
Each half barb represents 5 knots. Red represents wind gusts. In this
case, you have winds from the SSW at 25 knots with gusts to 30 knots.

So for example, these:

The one on the left represents wind of 15 knots gusting 20, and the one on the right is 15 knots gusting 25.
Odd ones like this:

Mean 5 knots gusting 10. Why they don't overlay I'm not sure.

In the case if the bottom graphic, why isn't that one "5 gust to 15"? How would 5G15 be represented?

@RalphJ like this: https://imgur.com/a/vMK8VKG ?

@GrzegorzOledzki - Great answer Ron. Wouldn’t 5G10 be a half one barb at the end of the windvane with half of it being black and half of it being red?

@DeanF. Unfortunately I'm not entirely sure about this. I interpret it as a long line is always 10kts and a short one is always 5. You add up the long lines *10 and short *5 to get the speed. Black for steady, red for gusts... I can't find anything definitive when they are arranged like that.

My last comment was poorly worded on retrospection. My understanding of the gust and steady barbs would be black for steady and red for gusts. Just like you suggested. My understanding of the wind barbs is that you add all of the lines and flags together to get the top range of the winds (gusts) regardless of color. You only count the black lines and flags for the sustained winds. The red lines and flags are the gust factor. With that logic, one line on the windbarb that is half red and half black is a gust of 10 with sustained winds at 5 knots (gust factor of 5).

A sustained of 5 with gusts up to 15 (a gust factor of 10) would  be a red, full-sized line at the end of the barb with a black half line before it. I have been checking the Aviation Weather Center graphic METAR for examples. I will post when I see one.


text:
What is the procedure to change an airport's published CTAF?
Similar to the problem experienced by the poster at Why do so many airports have the same CTAF frequency?, I have noticed that my CTAF is crowded. Not with local pilots, but with operations as much as 100nm away. There's little benefit to knowing that there are skydivers jumping in the mountains when I am an hour away in the plains. Likewise, hearing someone on downwind for an identically named-runway but on an island out in the ocean is, well, distracting.
I asked the airport manager what it would take and he chortled out, "An act of Congress." And he might well be right!
Does anyone know the procedure for getting a frequency changed? And can the 25kHz spacing be used or is that reserved for ATC and ATIS/AWOS/ASOS channels, which are, respectively, redundant or non-critical?

If your CTAF is a Unicom frequency, from AOPA:

the operator of the Unicom can change the frequency by submitting an application for modification on FCC forms 601 and 159 along with the $95 application fee. The current time for approval is typically 30 to 60 days from filing.

Frequencies are allocated by FCC, and rules are found at 4-1-11 of this document:

CTAF/Unicom/Multicom/FSS frequency allocation
Under this table there is a note which deals with both of your concerns, requesting a new frequency and usign the 25 kHz spacing:

In some areas of the country, frequency interference may be encountered from nearby airports using the same UNICOM frequency. Where there is a problem, UNICOM operators are encouraged to develop a “least interference” frequency assignment plan for airports concerned using the frequencies designated for airports without operating control towers. UNICOM licensees are encouraged to apply for UNICOM 25 kHz spaced channel frequencies. Due to the extremely limited number of frequencies with 50 kHz channel spacing, 25 kHz channel spacing should be implemented. UNICOM licensees may then request FCC to assign frequencies in accordance with the plan, which FCC will review and consider for approval.


text:
What is the fourth contact for on the Bose A20 headset's mic plug?
Bose mic plug
The Bose twin-plug cable for the A20 headset has a 4th contact.

In addition, the fourth section is rounded somewhat.

Standard GA mic plug
In comparison, the standard microphone plug only has three sections, and they're all smoothly cylindrical.

I can't quite figure out why Bose would have done it this way. Anyone have an answer?

This kind of plug has contacts, not pins, that one on the end of the mic plug's the PTT (Push To Talk) contact for a military spec microphone plug.

This helpful page explains it:

The first aircraft radios used carbon-granule microphones almost
identical to microphones used on telephones. However, a second
function was needed on the microphone in the form of a push-button to
key the transmitter and effect a changeover from receive to talk . . .
hence the name push-to-talk or PTT switch. Telephone companies not
only had pretty rugged microphones, they could offer a nifty 3-circuit
plug to go with it. To this day, the terms "tip", "ring" and "sleeve"
coined by the telephone industry are used to identify the three
circuits. The addition of a "ring" connection allowed us to craft a
PTT microphone companion to the already common headset. Being smaller
than a headset plug made it easy to avoid plugging the headset into
the wrong jack and vise-versa.
This particular plug is a mil-spec critter and appears to have yet
another "ring" between the tip and ring connections I've noted . . .
this may or may not be visible on your particular microphone plug . .
. in this particular plug, it's simply a spacer to fill out the shape
an maintain commonality with the dimensions of other style plugs.

When you watch some WWII movies you see pilots touching their mics or boxes close to their neck to talk, that's because the PTT was wired in-line with the headset rather than being on the control column or throttle. The 'tip' was there to enable that, when the PTT was depressed an electrical connection was made between the tip and the sleeve, signalling the radio to switch to talk mode. Note that the movies aren't always right, some airplanes had a PTT button off the headset, but that's just one of many common inaccuracies.
Nowadays the PTT button is often wired on the other side of the plug so that the PTT button goes to the stick or yoke, but the principle is the same, it's just that the connection is made farther up the line.
The plug on your headset is a slightly older design with a spacer between the tip and the ring to ensure the two stay separate electrically. It isn't an actual contact because it doesn't go anywhere.

I've never been in a GA plane or held a Bose headset. Am I correct when I say there are two plugs, one for the earphones (speaker) and one for the microphone, and the microphone plug is TRRS—but the second ring is a "dummy" or "spacer" and does not have an electrical function? I know that modern consumer earbuds use a TRRS plug where the tip and first ring are speaker signal, the second ring is ground, and the "sleeve" (essentially a fourth ring, by dimension) is the microphone signal.

Most GA mic plugs have three contacts, not four @randomhead, without a spacer. Typically they will have a slightly thicker non-conductive ring between the TIP and the RING, and a modern point shape. The Bose design is rare, I'm not sure why they chose it. maybe to conform to a military contract?


text:
May a Recreational Pilot demonstrate her/his own plane to a prospective buyer?
FAR 61.101(e)(12) states a recreational pilot may not

demonstrate that aircraft in flight as an aircraft salesperson to a prospective buyer;`

(Please note use of the world salesperson.)
AOPA's website publishes a question, presumably taken from the FAA knowledge test bank, which reads

Under what conditions, if any, may a recreational pilot demonstrate an aircraft in flight to a prospective buyer?
A) The buyer pays all the operating expenses.
B) The flight is not outside the United States.
C) None

According to AOPA (and other sites which publish the identical question), the answer is C) None.
So we can conclude the FAA knowledge test is saying that a Recreational Pilot may never demonstrate an airplane to a prospective buyer, no matter who is the plane's owner.
Is a Recreational Pilot de facto considered a salesperson when wishing to sell his/her plane? A lay definition of salesperson is someone whose job is to sell something. Certainly a private, non-dealer owner could not be considered to be "someone whose job...".
But perhaps the FAA has defined salesperson more broadly, or the test bank question could be erroneous (it wouldn't be the first time an official answer is subtly wrong).

I have deleted my answer because Pondlife bested me, and it would take to much editing to be accurate and competitive, but I do feel compelled to comment on my point that it is important to include a complete quote from the regs for the meaning to be correct.  Case in point:  "...a recreational pilot may not act as pilot in command of an aircraft to demonstrate that aircraft..." Acting as PIC is different than performing a demo, and your omission of the first part changed the meaning.  Good question though, and I'm glad you got your answer.

What if you were simply taking a new friend for a purely recreational ride in your aircraft, and they shortly after decided to buy it, in a wholly unrelated turn of events? (Seems this isn't actually a problem, but if it were, I think it'd be terribly hard to enforce)

@TCooper, there's nothing to enforce, because that's ok.  Read the answer below.  Besides, in your example it was, as you say, a purely recreational flight, not a demo.  You can't retroactively recategorize it.

It looks like a recreational pilot can demo their own aircraft, because the FAA considers "salesperson" to be a profession or job in the lay sense that you mentioned.
This is from the Federal Register:

The FAA maintains that, for sales demonstrations that are not
conducted by an aircraft salesperson, a sport pilot or a recreational
pilot can conduct this activity. Therefore, to ensure that
recreational pilots have at least the same privileges as sport pilots,
the FAA is revising § 61.101(d)(12) to allow a recreational pilot to
conduct sales demonstration flights as long as the pilot is not acting
as an aircraft salesperson.

The original wording of 61.101(d)(12) was:

To demonstrate that aircraft in flight to a prospective buyer

The current wording you quoted is:

to demonstrate that aircraft in flight as an aircraft salesperson to a
prospective buyer

In other words, they added "as an aircraft salesperson" to clarify that the restriction only applies to salespeople, not to all recreational pilots.
That means the question you quoted is apparently wrong. As to why it's wrong, I have no idea. The FAA doesn't actually publish its test question banks so many "FAA questions" out there aren't official at all.

Good additional info from the Federal Register regarding intent, and a good answer!  I thought I had this one in the bag, but now I'm contemplating deleting mine.  I think there are some valid points in it that may be useful, but now it isn't really accurate or correct...

@Pondlife thanks for the excellent answer, most particular the dive into the prior wording to show intent and not accident. Regarding the test bank, I had thought that the FAA's test banks were publicly accessible, under FOIA. Is this no longer the case?

@MichaelHall Thanks! I wasn't convinced that the regulation meant that a recreational pilot could demo an aircraft only if they weren't PIC. That seemed pretty convoluted to me, even for the FAA :-)

@KennSebesta I have no idea, I just know that it's 'common knowledge' that the banks aren't available (you can find lots of search hits about it on forums etc.). I have no idea whether they'd be subject to FOIA or not.


text:
What are the black boots on this Extra 300's prop?
I normally think of black boots on a prop as being for de-icing. However, the Extra 300 is not suitable for known-icing conditions, or even IMC flight. (It's AFM says "Only VFR flights at day are allowed").
Considering the Extra 300's aerobatic mission, the gyroscopic loads on the engine are fierce. The upshot is that the manufacturer would not include a single extra gram more than necessary here.
So why are they there/what do they do? It has the owner and I stumped.

Since you're in contact with the owner, they can touch and feel the black section -- does it feel like plastic/rubber, or paint?

Interesting question. Doing a Google image search for extra 300 propelller shows that many (but not all) have the same thing.


text:
What FAA legal interpretation clarifies that glider sight-seeing operations do not need a drug testing program?
I came across this once, and despite searching with Bing and Google cannot find it again. As memory serves, the FAA counsel wrote a letter finding that glider sight-seeing operations do not need approved drug testing programs.
All help is appreciated!

@RTO Good question, but alas I don't know. It was something I read in passing a while back, and I was thinking to send it to a friend who is getting his Part 135 for turboprop charter flights.

@RTO, Thank you, I didn't know about the FAA Interpretations Search site (https://www.faa.gov/about/office_org/headquarters_offices/agc/practice_areas/regulations/interpretations). The Fields interpretation was the one I was looking for, esp. re the conclusion that 91.147 does not apply, and therefore there is no need for a LOA or drug testing program. Do you want to turn your comment into an answer so I can give it the nod?